The invention relates to a process for the preparation of an aromatic hydrocarbon mixture having a high benzene content from non-aromatic organic compounds.
Benzene is an important base material in the chemical industry for, inter alia, the preparation of styrene via alkylation with ethene and dehydrogenation of the ethyl-benzene produced. An important source for the production of benzene is the hydrocarbon fraction boiling in the gasoline range which is obtained as a by-product in the preparation of ethene by thermal cracking of hydrocarbon oils, such as naphtha and gas oil. This hydrocarbon fraction, generally referred to as pyrolysis gasoline, has a very high benzene content. Extraction of the pyrolysis gasoline with a selective solvent for aromatics, such as sulfolane or diethylene glycol, yields an aromatic extract which, in addition to benzene, contains, inter alia, toluene and xylenes. Benzene may be isolated from this extract by distillation. The maximum quantity of benzene that can be produced in this way is dependent on the quantity of pyrolysis gasoline available, and thus on the demand for ethene. A group demand for benzene and an unchanged demand for ethene may give rise to problems in the supply of benzene. Since in the preparation of ethene by thermal cracking of hydrocarbon oils, not only aromatic by-products but, also, considerable quantities of non-aromatic by-products are formed, the applicants have carried out an investigation into the possibility of preparing benzene in an economically justified way by catalytic conversion of the latter by-products of ethene preparation. One of the motives for carrying out the investigation was the fact that new crystalline metal silicates having a special structure have recently been synthesized, which silicates show a high catalytic activity in the conversion of non-aromatic organic compounds into aromatic hydrocarbons, as described, for example, in U.S. Pat. Nos. 3,843,740, 3,843,741, 4,097,367, 4,238,318 and U.K. Pat. No. 1,555,928.
It has been found that if these silicates are used as the catalyst, an aromatic hydrocarbon mixture is obtained in which the aromatics substantially contain fewer than 12 carbon atoms, irrespective of the number of carbon atoms present in the organic compound used as the feed. The crystalline metal silicates concerned are characterized in that after one hour's calcination in air at 500.degree. C. they have the following properties:
(a) thermally stable up to a temperature of at least 600.degree. C., PA1 (b) an X-ray powder diffraction pattern in which the strongest lines are the four lines listed in Table 1: PA1 (c) in the formula which represents the composition of the silicate, expressed in moles of the oxides, and which, in addition to SiO.sub.2, comprises either Al.sub.2 O.sub.3 or Fe.sub.2 O.sub.3, the SiO.sub.2 /Al.sub.2 O.sub.3 and SiO.sub.2 /Fe.sub.2 O.sub.3 molar ratios are higher than 10. PA1 (a) the C.sub.4 fraction (BBB) PA1 (b) the C.sub.4 fraction, after separation of at least part of the butadiene (after complete separation of butadiene the remaining part is referred to as BB) PA1 (c) the pyrolysis gasoline PA1 (d) the C.sub.5 fraction of the pyrolysis gasoline PA1 (e) the C.sub.5 fraction of the pyrolysis gasoline after separation of at least part of the diolefins (isoprene, cyclopentadiene and dicyclopentadiene) PA1 (f) the C.sub.6.sup.+ fraction of the pyrolysis gasoline PA1 (g) the pyrolysis gasoline or its C.sub.6.sup.+ fraction after separation of at least part of the aromatics (for instance, the benzene/toluene/xylene fraction) PA1 (h) mixtures of one of the fractions mentioned under (a) and (b) with one of the fractions mentioned under (c)-(g).
TABLE 1 ______________________________________ d(.ANG.) Relative Intensity ______________________________________ 11.1 .+-. 0.2 Very Strong 10.0 .+-. 0.2 Very Strong 3.84 .+-. 0.07 Strong 3.72 .+-. 0.06 Strong ______________________________________
In the present patent application a crystalline silicate having a thermal stability of at least t.degree. C. should be taken to be a silicate whose X-ray powder diffraction pattern remains substantially unchanged upon heating to a temperature of t.degree. C.
The investigation carried out by applicants has shown that conversion of non-aromatic organic compounds over the aforementioned crystalline iron or aluminum silicates as catalysts does result in a hydrocarbon mixture having a high content of aromatic hydrocarbons of fewer than 12 carbon atoms in the molecule, but that these aromatic hydrocarbon mixtures comprise but little benzene.
Continued research carried out by the applicants has shown, however, that if the crystalline metal silicates having the special structure of the iron or aluminum silicates mentioned before, when used as catalysts for the conversion of non-aromatic organic compounds into an aromatic hydrocarbon mixture, contain both iron and aluminum and when, moreover, these metals are present in the silicates in such proportions that in the formula which represents the composition of the silicate, expressed in moles of the oxides, the SiO.sub.2 /Fe.sub.2 O.sub.3 molar ratio is 100-300 and the SiO.sub.2 /Al.sub.2 O.sub.3 molar ratio is 135-1900, result in an aromatic mixture with enhanced benzene content. This discovery is regarded as highly surprising in view of the disappointing results with respect to the production of benzene when using closely related crystalline metal silicates containing either only iron or only aluminum as the trivalent metal.